This invention relates to fluorescent lamps and more particularly to electrodeless, inductively coupled, low-pressure fluorescent lamps. Still more particularly, it relates to a shield having reduced electrostatic coupling between a high voltage source and the lamp.
Inductively coupled, low-pressure fluorescent lamps are known and are commercially available. These lamps provide high efficacy, long life and high lumen output. However, current electrodeless lamps operate in the frequency band near 2.6 MHz and have been limited to a re-entrant cavity configuration primarily due to common mode electromagnetic interference (emi) regulations. (In the United States these regulations are promulgated by the Federal communications Commission). EMI occurs when voltage applied to an induction coil capacitively couples to the discharge (which is maintained by the current flowing through the induction coil). Various shields have been successfully applied to reduce emi. Such shields are shown in U.S. Pat. Nos. 4,727,295; 5,325,018; and 5,726,523. These patents all disclose Faraday shields that reduce emi for a 2.65 MHz induction lamp wherein the lamps are low-pressure discharge lighting and have in common a geometrical configuration wherein a solenoidal induction coil that drives the discharge is located within a re-entrant cavity. In this geometry the shield material is primarily parallel to the magnetic filed lines and attenuation of the time varying magnetic flux is relatively limited as long as the shield does not form a closed loop that surrounds the flux enclosed by the solenoid. A large part of the magnetic flux, which induces the electric field that maintains the discharge, does not encounter the shield material and thus limits on shield thickness are not very stringent. The aforementioned U.S. Pat. No. 4,727,295 mentions a shield thickness of 0.25 mm (2.5xc3x97106 angstroms, hereinafter, xc3x85). U.S. Pat. No. 5,726,523 suggests a thickness of 1 mm (107 xc3x85); while U.S. Pat. No. 5,325,018 does not define a shield thickness except to say it may be very thin. U.S. Pat. No. 6,056,848 discusses a xe2x80x9cthinxe2x80x9d shield for a plasma reactor used for processing semiconductor substrates, such shields having a preferred thickness between 0.1 micron and 5 microns.
A shield based on the principles disclosed in the first three patents is limited to use with a solenoidal coil within a re-entrant cavity, thus having the practical effect of restricting most inductively coupled lamps driven at 2.65 MHz to a re-entrant geometry configuration.
It is, therefore, an object of this invention to obviate the disadvantages of the prior art.
It is another object of the invention to enhance the operation of electrodeless fluorescent lamps.
Yet another object of the invention is the provision of a method of operating an external coil electrodeless fluorescent lamp (ECEFL) with increased efficacy.
These objects are accomplished, in one aspect of the invention, by the provision of an inductively-coupled, electrodeless fluorescent lamp comprising: a lamp body having two opposed sides; an induction coil on one side of said body; and a magnetically transparent electrostatic shield interposed between said induction coil and said one side of said body, said shield comprising an insulating substrate; an electrically conductive layer on said substrate including means for reducing capacitive coupling between a voltage on said induction coil and a plasma discharge within said lamp body, said electrically conductive layer having a thickness between 400 xc3x85 and 1000 xc3x85, inclusive.
While this shield can be used in a re-entrant cavity lamp, its primary appeal is that it can be used with an induction coil of virtually any geometry that is external to a discharge vessel, which discharge vessel can also have virtually any geometric configuration.
A method of increasing the efficiency of an inductively-coupled, electrodeless fluorescent lamp is also provided. The method comprises the steps of; providing a lamp body having two opposed sides; positioning an induction coil on one side of said body; and positioning a magnetically transparent electrostatic shield between the induction coil and the one side of the body. The shield comprises an insulating substrate and has an electrically conductive layer thereon. The substrate including means for reducing radio frequency capacitive coupling between a voltage on said induction coil and a plasma discharge within the lamp body. The electrically conductive layer has thickness between 400 xc3x85 and 1000 xc3x85, inclusive. The lamp is operated by inducing an operating voltage on the lamp through the induction coil.